1. Field of the Invention
The invention relates generally to frequency conversion systems, devices, and methods, and more specifically to radio frequency communication devices and systems including, mixers, radio tuners, transmitter, and receivers incorporating FET mixer type frequency conversion devices for up- and down-frequency conversion.
2. Background of the Invention
Conventional heterodyne receivers down convert a radio-frequency (RF) signal to a baseband signal using one or more intermediate stages in which the RF signal is converted to one or more intermediate-frequency signals, lower than the RF signal, until the base-band frequency is reached. A heterodyne transmitter generates a higher frequency RF signal from a baseband signal using one or more intermediate stages to up-convert the frequency. A transmitter provides both transmit and receive components and function.
In simplified terms, a homodyne receiver directly down-converts radio-frequency (RF) signals to baseband frequency without intermediate stages. Analogously a homodyne transmitter up-converts from base-band to RF without intermediate stages. A radio system (frequency conversion stage, tuner, receiver, transmitter, or transceiver) may include homodyne and heterodyne components. In this disclosure the term system may be used when referring to any or a combination of such stage, tuner, receiver, transmitter, or transceiver, so as to simplify the description.
Conventional homodyne systems may typically have a poor dynamic range, unacceptably high distortions for some applications, and other undesirable characteristics as compared to non-homodyne systems. The poor dynamic range is typically the result of at least two significant factors. First, distortions, including input second order intercept point (IP2), input third order intercept point (IP3), and so-called xe2x80x9cNxc3x97Nxe2x80x9d distortions, cause unwanted spurious responses to fall within the frequency band of interest. Second, amplitude and/or phase imbalances contributed by an imperfect quadrature local oscillator, may cause errors in the in-phase channel (I) or quadrature-phase channel (Q) signals before they are digitized by the analog-to-digital converter (ADC) in the digitizer, resulting in non-linearities in the conversion process. (These quadrature or I/Q channels are sometime referred to as sine and cosine channels or signals as a result of the out-of-phase relationship between the channels and the manner in which they are conventionally generated.) These non-linearities directly or indirectly result in distortion and loss of useful dynamic range.
These conventional homodyne systems may beneficially employ software algorithms, residing in the Digital Signal Processing (DSP) section of the tuner, transmitter and/or receiver down the signal path from the homodyne frequency conversion stage, to compensate for some of the distortions, errors, and other anomalies in the such conventional homodyne systems (especially receivers) with minimal success, but this additional DSP task undesirably requires a higher clock rate than would otherwise be required for a given bandwidth. Wider signal bandwidth may typically need a processor clock rate that is from about 10 times to about 20 times or more the clock rate required without compensation, in order to compensate phase and amplitude errors over the entire receiver bandwidth of interest. The higher clock rate presents additional problems in itself. Digital compensation after digitization reduce the wanted spectrum bandwidth. Without compensation, homodyne receivers or direct conversion receivers employing mixers are limited to around 40 dB of dynamic range and bandwidth in the audio frequency range.
The trend in new radio systems technology receiver/tuner development is predicted to concentrate on moving the RF spectrum down to baseband frequencies where it will be digitized and processed under software control. This will impose even more stringent demands for dynamic range, increased sensitivity, and lower distortion. Reducing size, weight, and power consumption to provide longer operating times under battery power, are also concerns for commercial and non-commercial applications. A key system performance challenge involves keeping the spectrum dynamic range (sensitivity vs. distortion) as high as possible before digitization in the ADC while maintaining high sensitivity and controlling distortion.
An additional problem with conventional wireless (radio) communication systems pertains to frequent requirements for skilled radio operators to initiate and maintain contact between multiple radio stations or transceivers. This problem is particularly acute because of the need to monitor or provide surveillance over a large HF/VHF/UHF frequency spectrum. Both commercial and non-commercial communicators have been working to achieve automatic, reliable and robust communications using the HF/VHF/UHF spectrum, particularly the HF spectrum. One goal of this work has been an attempt to eliminate or reduce the need for highly skilled radio operators while simultaneously increasing the reliability of the HF spectrum as a communication medium.
Automatic Link Establishment (ALE), also known as Adaptive HF, is an integral part of this effort. ALE is defined as the capability of an HF radio station to make contact between itself and another station or stations under automatic processor control. ALE techniques include automatic signaling, selective calling, and automatic handshaking at different bands in the HF spectrum. Monitoring and following all these activities requires a near simultaneous full band HF receiver. Digitizing the entire HF frequency band, and handling ALE protocol with Digital Signal Processing (DSP) presents many challenges. For example, if the monitoring sites are not ideal in location, dynamic range, resulting from near by transmitters masking far away ALE signals, presents a problem. It has been estimated that an adaptive HF monitoring solution requires full simultaneous HF coverage with 100 dB of Spur Free Dynamic Range (SFDR). The cost for implementing and deploying such ALE systems also remains problematic.
This and other performance challenges have been addressed in part by the development of analog-to-digital converters (ADCs) which have increased resolution (sensitivity), increased Spur Free Dynamic Range (SFDR), and greater baseband spectral bandwidth. ADCs having 14-bit resolution and 30 MHz baseband bandwidth, and which can be clocked out at 65 mega samples per second (MSPS), with a projected SFDR of 85-90 dB or more are available and narrower bandwidth ADCs (for example, bandwidths less than about 10 Mhz) and providing 16-bit resolution at an even greater 95-100 dB SFDR are under development. These devices provide the needed ADC performance improvement over earlier 12-bit ADCs. Even though higher-performance ADCs have been developed, other problems remain.
Frequency conversion or mixer stages in conventional RF systems have heretofore been unable to attain the approximately 85-100 dB Spur-Free Dynamic Range required in certain tuner/receiver systems, particularly where the output of that mixer stage was intended as the input to high performance Analog-to-Digital Converters (ADCs) where the 100 dB SPRD, is required at the input. In fact such systems have been limited to substantially lower performance. The last or final mixer stage just prior to output to the ADC (baseband frequency converter stage) typically has the highest signal amplitude level in the tuner. A state-of-the-art ADC requires about a 2 volt peak-to-peak signal for full ADC conversion scale, and should have all spurious signal products down by about 100 dB in order to utilize the capabilities of the ADC without introducing other undesirable artifacts. These ADC performance specifications correspond to a baseband spectrum mixer stage coupled to the ADC input terminals having an input third order intercept point (IP3) of about +50 dBm and an input second order intercept point (IP2) of about +100 dBm.
Another problem in conventional tuners is that they typically perform the final stage mixing to baseband at a low Intermediate Frequency (IF) signal amplitude level, and then boost the amplitude of the final stage mixer output with a separate power amplifier to achieve the desired ADC signal level (typically in the range of from about 1 Volt to about 4 Volts peak-to-peak). This baseband conversion approach only achieves approximately +43 dBm IP3 and +82 dBm IP2 into the ADC, may have unacceptable levels of distortion, and typically may not provide performance levels that keep with evolving state-of-the-art ADC dynamic range capability, or that meet the needs the end user.
In addition to the above described final mixer stage problems in conventional tuners, the first conversion stage of a tuner also significantly affects overall dynamic range, in fact so much so that degradation in the first stage may make it impossible to meet overall system performance requirements. For example, if the first mixing operation is too lossy, there may be an added requirement for a preamplifier gain stage in the signal path to boost the signal amplitude in an attempt to achieve the required sensitivity. However, such a preamplifier gain stage in the signal path upstream from the mixer circuit undesirably increases the required IP3 and IP2 of the mixer by an amount equal to the added preamplifier gain. Therefore, any system requirement for a preamplifier gain stage to increase sensitivity imposes even more severe constraints on other radio system components. Desirably, a mixer would have very low conversion loss in the first stage to avoid the need for any preamplification, and also have a high or large dynamic range.
One mixer known to the inventors is marketed by Steinbrecher under the name xe2x80x9cPARAMIXERxe2x80x9d and is purported in advertising materials to provide a mixer having an input third order intercept point (IP3) on the order of about +50 dBm and an input second order intercept point (IP2) on the order of about +100 dBm, but the inventors have not verified or confirmed such performance claims. In spite of its purported IP3 and IP2 performance figures, even this Paramixer has disadvantages. For example, such conventional mixers are relatively large (on the order of about 100 cubic inches), power inefficient (about 8 watts input power is needed to process 100 milliwattsxe2x80x94an efficiency of only about two percent (2%)), and expensive (for example, costing up to several hundred dollars per mixer).
Therefore, although some progress has been made in improving mixer performance to achieve high quality radio systems, such developments have not solved the need for compact, small form factor, light weight devices, having lower power consumption, long battery life in battery powered devices, at moderate to low cost.
Therefore, there remains a need for high-performance radio systems including tuner, receiver, and/or transmitter components that are low-cost, compact, and energy conservative, particularly for mobile or hand-held applications. There also remains a need for mixing devices for frequency conversion generally, and more particularly for use in these high-performance radio systems, surveillance systems, and instrumentation systems.
In one aspect, the invention includes a wireless communication structure, device, and system and method for operating the same, a mixing structure for use with the wireless communication device or for use otherwise and a method of mixing signals, as well as an inventive differential square wave mixer switching circuit and method for controlling the mixer device which may be used with the inventive mixing device and wireless communication device.
The inventive mixer structure and method (also referred to as a xe2x80x9csuper-mixerxe2x80x9d because of its superior (e.g. super) properties as compared to conventional mixers) include an overall mixer architecture topology and several embodiments of the mixer structure which present variations particularly suitable for use in a radio receivers, transmitters, tuners, as well as instrumentation systems, and other systems and devices performing frequency conversion. The inventive mixer is applicable to homodyne and heterodyne receiver/transmitter/tuner implementations, instrumentation and telemetry systems. The invention also provides structural and methodological components of the mixer including a precise mixer device within its LO phase splitter, and differential square wave gate drive. Wireless communication devices includes radios, cellular telephones, and telemetry systems whether land, sea, airborne, or space based, and whether fixed or mobile.
The inventive mixer device is advantageously a GaAs FET mixer where the FETs are implemented on a common substrate. The inventive mixer has superior intermodulation and harmonic distortion suppression and features excellent conversion loss, noise figure, port match, and port isolation as a result of its topology. The mixer device circuit combines the advantages of series mixing FETs, a triple balanced design using a balanced passive reflection transformer, a very precise LO phase splitter, and square wave gate drive to achieve its high levels of performance. It is power conservative and offers the advantage of long battery life in portable devices such as portable radios and cellular telephones as it requires only a modest amount of DC and LO drive power, and is useful for operation over at least a multi-decade bandwidth.
Therefore, one object of the invention is to provide a high-performance mixing device that achieves a high IP2 and IP3.
Another object of the invention is to provide a mixer device which is energy conservative, power efficient, and which therefore provides size, weight, and operational life for mobile and/or portable hand-held implementations.
A further object of the invention is to provide a mixer having a large dynamic range and very low distortion.
Another object of the invention is to provide a differential square wave drive circuit for use with a mixer.
Yet another object of the invention is to provide a mixer generating, at most, very low spurious signal withing the frequency band of interest.
Still another object of the invention is to provide a mixer generating an analog output signal that does not exceed the input specifications for 14-bit and higher bit analog-to-digital converters.
Another object of the invention is to provide a mixer that minimizes amplitude and phase imbalances, such as imbalances contributed by an imperfect quadrature modulator local oscillator frequency signal.
Still another object of the invention is to provide a mixer that meets or exceeds the input requirements of high-bit (e.g. 12-bit, 14-bit, 16-bit) analog-to-digital converters (ADCs) so as to provide a radio in which the mixer is employed where the RF spectrum may be moved down to baseband frequencies, while maintaining high sensitivity and low distortion, where the spectrum may be digitized by the ADCs and processed under software control.
An additional object of the invention is to provide a high-performance direct conversion system.
Another object of the invention is to provide a radio, tuner, receiver, and/or transmitter, or components thereof in which about 100 dB or greater of Spur Free Dynamic Range is provided over a sufficiently large spectral bandwidth meeting the needs of an Automatic Link Establishment (ALE) or adaptive HF, VHF, and/or UHF system.
A final object of the invention is to provide a method for mixing signals to provide a high performance mixer achieving high SPDR, large bandwidth, low distortion, and low power consumption in a wireless communication system.
Additional objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the drawings.